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Stoichiometry Notes
Stoichiometry
• By combining our abilities to balance equations and do simple unit conversions, we can now complete stoichiometry problems (mass to mass conversions)
Particle and Mole Relationships
• Chemical reactions stop when one of the reactants is used up.
• Stoichiometry is the study of quantitative relationships between the amounts of reactants used and amounts of products formed by a chemical reaction.
Particle and Mole Relationships (cont.)
• Stoichiometry is based on the law of conservation of mass.
• The mass of reactants equals the mass of the products.
Particle and Mole Relationships (cont.)
Particle and Mole Relationships (cont.)
• A mole ratio is a ratio between the numbers of moles of any two substances in a balanced equation. Refer to coefficients in the balanced equation to determine the mole ratio.
Stoichiometry
• Stoichiometry problems consist of predicting the amount of grams of product that will form from a reaction if you know how many grams of reactant you have, or predicting the number of grams of reactant that will be needed to produce a certain amount of product.
Stoichiometry
• As an example, let’s consider a ham sandwich
• To make a ham sandwich, here is the equation:
• 1 H(am) + 2 B(read) + 1 L(ettuce)1 HB2L (also known as a Ham Sandwich)
Stoichiometry
• If you had 4 slices of ham, 6 pieces of lettuce, and 7 slices of bread, how many ham sandwiches could you make?
(remember 1 H + 2 B + 1 L 1 HB2L)
This problem is the same as a stoichiometry problem.
Stoichiometry
• How about how many pieces of bread would you need to make 14 ham sandwiches?
• This is another type of problem that you’ll do in stoichiometry
Using Stoichiometry
• All stoichiometric calculations begins with a balanced chemical equation.
4Fe(s) + 3O2(g) 2Fe2O3(s)
Using Stoichiometry (cont.)
Using Stoichiometry (cont.)
• Steps to solve mole-to-mole, mole-to-mass, and mass-to-mass stoichiometric problems
1. Complete Step 1 by writing the balanced chemical equation for the reaction.
2. To determine where to start your calculations, note the unit of the given substance.
• If mass (in grams) of the given substance is the starting unit, begin your calculations with Step 2.
• If amount (in moles) of the given substance is the starting unit, skip Step 2 and begin your calculations with Step 3.
Using Stoichiometry (cont.)
3. The end point of the calculation depends on the desired unit of the unknown substance.
• If the answer must be in moles, stop after completing Step 3.
• If the answer must be in grams, stop after completing Step 4.
Stoichiometry
• We will solve these problems using a “T” chart just like we did for unit conversions, but we will add additional cells
Given info Units to convert into Units to convert into etc
Units to cancel Units to cancel etc
and we use additional conversion factors
• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/limitr15.swf
• How would you determine how many ham sandwiches you could make if you had 16 g of bread?
• Ham = 2.1 g per slice
• Lettuce = 1.8 g per slice
• Bread = 2.4 g per slice
Stoichiometry
• The additional factors we will use include:
1. ___ moles of Y = ___ moles of Z
where the coefficients in a balanced equation gives us the numbers that replace the blank spots
Stoichiometry
2. 1 mole = (molar mass) # of grams3. Density (# of grams/L) = 1 Liter of solid,
liquid, or aqueous solution (or 0.001 L if density is given in mass/mL or mass/cm3)
4. 22.4 Liters of any gas (at STP)= 1 mole5. 1 mole = 6.022 x 1023 parts (atoms, ions,
molecules, etc)STP = standard temperature and pressure
Stoichiometry
grams X A moles X C moles Y A grams Y
D E B B E D
liters X atoms X atoms Y liters Y
A: 1 mole = (molar mass) # of grams
B: 1 mole = 6.022 x 1023 parts (atoms, ions, molecules, etc)
C: (Coefficient of balanced equation) moles X = (Coefficient of balanced equation) moles Y
D: Density: (# of grams) = 1 Liter of solid, liquid, or aqueous solution (or 0.001 L if density is given in mass/mL or mass/cm3)
E: 22.4 Liters of any gas (at STP)= 1 mole
Stoichiometry
• Practice problems p. 375, 376, 377
Limiting and Excess Reactants
• Let’s return to the ham sandwich example.
(remember 1 H + 2 B + 1 L 1 HB2L)
• If you have 4 pieces of ham, 6 pieces of lettuce, and 7 slices of bread, how many leftover pieces of ham and lettuce will you have?
• This is known as a limiting reactant problem
Limiting and Excess Reactants
• Here’s another example:
Why do reactions stop?
• Reactions proceed until one of the reactants is used up and one is left in excess.
• The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed.
• The excess reactants are all the leftover unused reactants.
Why do reactions stop? (cont.)
• Determining the limiting reactant is important because the amount of the product formed depends on this reactant.
Limiting and Excess Reactants
• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/limitr15.swf
Limiting and Excess Reactants
• Once the limiting reactant has been determined, you can use it to calculate how much of the other reactants will be consumed, and how much of the products will be produced.
• To determine how much will be left over, simply subtract the consumed/produced amounts from the available amounts.
Calculating the Product when a Reactant is Limiting
• S8(l) + 4Cl2(g) → 4S2Cl2(l)
• 200.0g S8 and 100.0g Cl2
• Determine which is the limiting reactant
Calculating the Product when a Reactant is Limiting (cont.)
• Calculating the amount of product formed
Calculating the Product when a Reactant is Limiting (cont.)
• Determine the amount of the excess reactant
Calculating the Product when a Reactant is Limiting (cont.)
• Using an excess of a reactant can speed up the reaction.
• Using an excess of a reactant can drive a reaction to completion.
Limiting and Excess Reactants
• Do practice problems #23 and 24 on pg. 383
Limiting and Excess Reactants
Stoichiometry Pizza Exercise (on handout)
• Find out how many pizzas you can make using the amount of each ingredient
• Whichever ingredient makes the least pizzas is the limiting reactant.
How much product? (cont.)
• The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.
• The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.
• The percent yield of a product is the ratio of the actual yield expressed as a percent.
Percent Yield• You can also use stoichiometry
calculations to determine percent yield in a reaction.
• The amount that you actually get may be more or less than the amount that you would predict in a stoichiometry problem.
• If you divide the actual yield by the theoretical yield (predicted), you can determine the % yield.
Percent Yield in the Marketplace
• Percent yield is important in the cost effectiveness of many industrial manufacturing processes.
• Laboratory reactions do not always produce the calculated amount of products.
• Reactants stick to containers.
• Competing reactions form other products.
Percent Yield
• Do practice problems #29, 30 on p. 387
